Optical objective



May ll, 1943.

Search Roem SR D A. wARMlsHAM Erm. 2,319,171

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OPTICAL OBJECTIVE Filed Dec. 15, 1941 2 Sheets-Sheet 2 4 R5 R6 R7 *.5575 Pg -2444 -4/50 +2950 'u luv' Patented May 11, 1943 OPTICAL OBJECTIVE Arthur Warmisham and Charles Gorrie Wynne, Leicester, England, assignors to Taylor, Taylor & Hobson Limited, Leicester, England, a company oi Great Britain Application December 15, 1941, Serial No. 423,118 In Great Britain December 2, 1940 17 Claims.

This invention relates to optical objectives for photographic or like purposes, comprising two or more divergent elements 'and two or more convergent elements, and corrected for spherical and chromatic aberrations, coma, astigmatism, curvature of field and distortion, and having small zonal spherical aberration.

It is well-known to provide paraxial chromatic correction in a doublet in respect of two colours, for example red and green, by the use of an appropriate combination of crown and flint glass, but owing to the different relative partial dispersions of the two kinds of glass the correction does not extend throughout the spectrum, andA there is a residual colour aberration known as secondary spectrum. Reasonably good correc tion can be obtained in the well-known triplet objective, which however does not provide correction for field. curvature or astigmatism.

The present invention has for its object to provide good correction for secondary spectrum in a photographic or like objective having small zonal spherical aberration without sacrificing correction for astigmatism, field curvature and distortion.

The necessary conditions can be expressed mathematically as follows. If fp and mp are respectively the focal length and the magnification of a lens element p having refractive indices ne nn ne nr ng respectively for the lines C D e F g Abbe v number V (np-1 p mr-nc and relative partial dispersion then good secondary spectrum correction is obtained if and for all the elements of the objective. It should be made clear that the magnification mp, herein referred to, may be defined as being equal to the ratio hp/hi, where hp and h1 are\respectively the ordinates of the points of intersection with the lens element p and with the first lens element of a paraxial ray of the wave-length of the D-line through the conjugate points for which the objective is corrected.

In the objective according to the invention one of the divergent elements is made of an alkaline halide crystal and the remaining elements are all made of optical glass. Preferably the other divergent element, or one of the other divergent elements, is made of a dense int glass, whilst at least one of the convergent elements is made of a glass having an Abbe v number less than 50.

It is to be understood that the terms front and rear are herein used to relate to the sides of the objective respectively nearer to and further from the longer conjugate in accordance with the usual convention.

Fig. 1 is a sectional view of an objective embodying one form of the invention.

Fig. 2 is a corresponding view showing a modified form of the invention.

Fig. 3 is a corresponding view showing another modification of the invention.

Fig. 4 is a, corresponding view showing another modication ofthe invention.

Fig. 5 is a corresponding view showing another modification of the invention.

Fig. 6 is a corresponding view showing another modification of the invention.

Fig. 7 is a corresponding View of another modification of the invention. x

The invention may be applied to photographic or like objectives of various types, and seven practical examples of objective according thereto are illustrated respectively in the seven figures of the accompanying drawings. Numerical data for these examples are given in the following tables in which R1 R2 represent the radii of curvature of the individual lens surfaces counting from the front, the positive sign indicating that the surface is convex to the front and the negative sign that it is concave thereto, D1 Da represent the axial thicknesses of the individual lens elements, and S1 Sz represent the axial lengths of the air gaps between the components. The tables also give the mean refractive indices ma, the Abbe u numbers, and the relative partial dispersions for the intervals (e to 9)/ (C to F) of the glasses or crystals used for the individual elements.

The rst two examples relate to objectives of the type comprising two compound divergent meniscus components having their concave surfaces facing one another and located between two simple convergent components.

I In this example sodium bromide is used for the rear divergent element and dense iiint glass for l the front divergent element, the two front convergent elements being of dense barium crown glass and the two rear convergent elements being of dense barium flint glass. This example gives a. very marked improvement in secondary spectrum correction over known objectives of similar type using glass throughout. Thus between the C and F lines the wave surface retardation is +.05 at the edge and .02). at an intermediate zone (.7 of edge aperture) as contrasted with -.93 and -.64 respectively for a typical objective using glass, and the corresponding figures betweenthe e and g lines are -.067\ and -.03}. as contrasted with +1.33). and +.39 Similar results can also be obtained with the use of potassium iodide or rubidium iodide instead of sodium bromide.

Instead of using the crystal for the rear divergent element, it may be used for the front divergent element, but in this case a crystal of lower refractive index, such as potassium bromide should preferably be used. Data for one such example are given in the following table:

In this example the two convergent components and the convergent elements of the second and third components are all made of dense barium int glass. The divergent element of the second component is made of potassium bromide crystal, and the divergent element of the third component is made of dense flint glass.

The third example is applied to an objective of the type comprising two compound divergent meniscus components located behind two simple convergent components and in front of a third simple convergent component.

Example III Equivalent focal length 1.000 Relative aperture f/1.4

R f Relattie Thickness or e. rac' Abbe v .par l. Radms separation mleem) number dlsgrwn D1= 0671 1. 613 59. 6 1. 00 R2=+L 653 Sn=l 002 R5=| 442 D3= 1474 1. 644 48. 3 1. 02 Rn= D4= 0305 1. 621 36. 1 1. 05 R1=-l 2661 S3= 2033 Rg= 3482 Ds= 0297 1. 6634 21. 3 987 Rt=1 169 D= .1474 1. 644 48. 3 1. 02 R1u= 4854 This example has potassium iodide for the rear divergent element and dense flint glass for the front divergent element, and gives a considerable improvement in secondary spectrum correction over known objectives of similar type using glass throughout. Thus between the C and F lines the wave retardation is .10 at the edge and +.107i at an intermediate zone (.7 of edge aperture) as contrasted with 1.91). and -1.13). respectively for a typical known objective, the corresponding figures between e and g lines being -l-.O'lx and +20). as contrasted with +2.34). and |.'l'l

The fourth example relates to an objective of the type comprising two simple divergent components located behind two simple convergent components and in front of a third simple convergent component.

This example uses sodium bromide for the divvv v' IIUU vergent fourth element and dense int glass for the divergent third element, whilst the convergent front and rear elements are made of dense barium flint glass and the convergent second element of dense barium crown glass.

The fifth example is of the type known as a telephoto objective wherein known examples using glass throughout have suffered from serious secondary spectrum. Such objectives usually consist of a pair of meniscus doublets with their concave surfaces facing one another, but in the present example the front component consists of a. divergent element cemented between a. front convergent element and a rear divergent element, the rear component consisting of a divergent element cemented in front of a con-' vergent element.

Example V Equivalent focal length 1.000 Relative aperture 175.6

Relative partiel dispersion U C- F Retractive index np vThickness or separation Abbo u Radius number This example uses potassium iodide crystal for the divergent middle element of the front component and dense i'lint glass for the divergent rear element of the front component as also for the convergent rear element of the rear component, the front elements of the first and second components being respectively made of light barium flint and crown glasses.

This example may be modified by employing the crystal for the rear element of the front component and the dense int glass for the middle element, or again by making the rear component of triplet construction and the front component of doublet construction with the crystal as one of the elements, preferably the middle element, of the rear component.

According to a further feature of the invention an objective of known type having at least one divergent element and two or more convergent elements is modified to aiord correction for secondary spectrum by replacing the divergent element (or one of the divergent elements) by two divergent elements compounded together, one

of such elements being made of an alkaline halide vUUlbH HU) Example VI Equivalent local length 1.000 Relative aperture i/l. 4

l i i R f Relatt'ie l Thickness' or e. 1:30' Ahbe 1 .par la Hmm air separation mlexcun number dlsrion l C- F D|= .1013 1. 516 04 l. .988 Rg= 1. 26T Y S1= 1. 305 l R1.'=+x. 447

D2= .1391 1. 6125 3T. 3 1.051 R4=+6- 031 Sr= 01.01 R5=+ 6427 D3= .2432 1.6125 59.6 .999 Rvi= D4= 1619 1. 6973 30. 5 1. 067 R5=+L -120 DF 172. 1. 6634 21. 4 988 R9=+ .3746

S4= .2343 R1o=+ 8120 Dt= 1391. 1. 6216 60. 2 998 R1|=1286 Example VII Equivalent focal length 1.000 Relative aperture f/1.4

. Thickness or e. mi" Aboe t Pa l. Radius tive dispersion air separation index "D number y Di=.1081 1. 6135 59. 6 999 R1=+ 7. 262

Dq=. 2453 1.5732 51. 9 1.012 R1=+ 3. 505

Da=. 1001 l. 7492 27.8 1. 078 R='+ 9102 S3=. 1747 Rg= 5236;

In the sixth example the objective comprises ve components of which the rst and fourth are divergent and the others convergent, the fourth component being compound and consisting of two divergent elements, whilst the remaining four components all consist of simple elements. Potassium iodide crystal is used for the rear element of the fourth component and dense flint glass for the front element thereof. Dense flint glass is also used for the convergent second component, the other two convergent components being made of dense barium crown glass, whilst barium silicate crown glass is used for the divergent front component.

In the seventh example the objective comprises a divergent compound component located behind two simple convergent components and in front of a third simple convergent component, the front and rear divergent elements of the divergent third component being made respectively of dense flint glass and of potassium iodide crystal. The convergent front component is made of dense barium crown glass, and the convergent second component of lght barium flint glass, whilst dense int glass is used for the convergent rear component.

` It will be appreciated that the foregoing examples may be modified in various ways within the scope of the invention, for instance by the use of other high index alkaline halide crystals in place of the potassium iodide crystal described.

What we claim as our invention and desire to secure by Letters Patent is:

1. An optical objective corrected for spherical and chromatic aberrations, coma, astigmatism, eld curvature and distortion, and having small zonal spherical aberration, and comprising at least two divergent elements and at least two convergent elements in axial alignment, one of the divergent ele nts being made of an alkaline halide crystallwhilst all the other elements of the objective are made of optical glass the objective approximately fulfilling th two equations wherein mp, fp vp and p respectively represent the magnification, the focal length, the Abb v number and the relative partial dispersion of an element p of the objective and the symbol E indicates algebraic summation of the expressions for all the elements of the objective.

2. An optical objective corrected for spherical and chromatic aberrations, coma, astigmatism, field curvature and distortion, and having small zonal spherical aberration, and comprising at least two divergent elements and at least two convergent elements in axial alignment, one of the divergent elements being made of an alkaline halide crystal and another of a dense flint glass, whilst at least one of the convergent elements is made of a glass having an Abbe v number less than 50 the objective approximately fulfilling the two equations wherein mp, fp vp and 0p respectively represent the magnification, the focal length, the Abb v number and the relative partial dispersion of an element p of the objective and the symbol E indicates algebraic summation of the expressions for all the elements of the objective.

3. An optical objective corrected for spherical and chromatic aberrations, coma, astigmatism, fleld curvature and distortion and having small zonal spherical aberration, and comprising two compound divergent meniscus components with their concave air-exposed surfaces facing one another each consisting of a convergent element cemented to a divergent element, and at least one convergent component on either side of the pair of divergent components, one of the dlvergent elements being made of an alkaline halide crystal, whilst all the other elementsof the objective are made of optical glass the objective approximately fulfilling the two equations wherein mp, fp vp and 0p respectively represent the magnication, the focal length, the Abb n number and the relative partial dispersion of an element p of the objective and the symbol E indicates algebraic summation of the expressions for all the elements of the objective.

4. An optical objective as claimed in claim 3. in which the other divergent element is made of a dense int glass and at least one of the convergentelements is made of a glass having and Abbe v number less than 50.

5. An optical objective as claimed in claim 3, in which the rear divergent element is made of an alkaline halide crystal having mean refractive index greater than 1.6 and the front divergen elementis made of dense flint glass.

6. An optical objective corrected for spherical and chromatic aberrations, coma, astigmatism, eld curvature and distortion, and having small zonal spherical aberration, and comprising two compound divergent meniscus elements with their concave air-exposed surfaces facing one another and at least one convergent component on either side of the pair of divergent meniscus components, one divergent component consisting of a divergent element made of an alkaline halide crystal, cemented to a convergent element made of dense barium flint glass, whilst the other divergent component consists of a divergent element made of dense iiint glass cemented to a convergent element made of dense barium iiint glass the objective approximately fulfilling the two equations Wal- @1.22:

fn "v 0 and 2f Vn 0 wherein mp, fp vp and 0p respectively represent the magnication, the focal length, the Abb v number and the relative partial dispersion of an element p of the objective and the symbol 2 indicates algebraic summation of the expressions for all the elements of the objective.

7. An optical objective as claimed in claim 6, in which the rear convergent component is made of dense barium flint glass.

8. An optical objective corrected for spherical and chromatic aberrations, coma, astigmatism. eld curvature and distortion, and having small zonal spherical aberration, and comprising four axially aligned components of which the outer pair are convergent and the inner pair each consist of a compound meniscus divergent cornponent having a divergent element cemented to a convergent element, one of the two divergent elements being made of an alkaline halide crystal and the other of a dense int glass, whilst at least one of the convergent elements is made of a glass having an Abbe v number less than 50 and the others are also made of optical glass the objective approximately fulfilling the two equations wherein mp, fp vp and 9p respectively represent the magnification, the focal length, the Abb v number and the relative partial dispersion of an element p of the objective and the symbol 2 indicates algebraic summation of the expressions for all the elements of the objective.

9. An optical objective as claimed in claim 8, in which the rear divergent element is made of an alkaline halide crystal having mean refractive index greater than 1.6.

10. An optical objective as claimed in claim 8, in which the convergent front component and the convergent front element of the second component are each made of dense barium crown glass and the divergent elements of the second and third components are respectively made of dense flint glass and of sodium bromide crystal, whilst the convergent rear element of the third component and the convergent rear component are each made of dense barium flint glass.

11. An optical objective as claimed in claim 8,'

in which the front divergent element is made o1' an alkaline halide crystal having mean refractive index less than 1.6.

12. An optical objective as claimed in claim 8.

in which the front divergent element is made of potassium bromide, and the convergent eiements are all made of dense barium flint glass.

13. An optical objective corrected for spherical and chromatic aberrations, coma, astigmatism, eld curvature and distortion and having small zonal spherical aberration, and comprising ve axially aligned components of which the front two components and the rear component are convergent and the third and fourth components each consist of a compound divergent meniscus component having a convergent element cemented to a divergent element, the rear divergent element being made of an alkaline halide crystal having mean refractive index greater than 1.64 whilst the front divergent element is made of dense flint glass the objective approximately fulfilling the two equations wherein mp, fp p and 0p respectively represent the magnification, the focal length, the Abb u number and the relative partial dispersion of an element p of the objective and the symbol E indicates algebraic summation of the expressions for all the elements of the objective.

14. An optical objective as claimed in claim 13, in which the front two convergent components are each made of dense barium crown glass, and the remaining convergent components are made of dense barium flint glass, the rear divergent element being made of potassium iodide crystal.

15. An optional objective having numerical data substantially as set forth in the following table:

Equivalent focal length 1.000

Relative aperture f/2 R f Relattitie Thickness or e. me' .Abbe v .par u? Radlus separation inleem) number dlsgrlon D1=.081 1. 6128 59.6 1.00 R2 +25. T5

D2=.151 1. 6128 59. 6 1. O R4: 1. 032-1 S2 1955 Rt: 3048 D5=. 1596 1. 6423 48. 3 1. 02 Rg= 4127 D6=.1015 1.6423 48.3 1.02 Rm= 9804 wherein RiRz indicate the radii of the individual surfaces counting from the front, D1D2 indicate the axial thicknesses of the individual elements and SiS'z indicate the axial air separations between the components.

Search Roel 16. An optical objective having numerical data. substantially as set forth in the following table:

Equivalent focal length 1.000 Relative aperture 172.0

R. Thickness or e. m0' Abbe v .par i Radius separation mvxenn number dlsgron D1=. 0845 1.644 48. 3 1.02 R1=+L 599 Da= 0418 1. 558 3l. 5 1. 00 Rs=+ 2781 S2= 1591 Ra= 3162 Sa= 0050 R9=l 3.167

Dc=. 0746 1. 644 48. 3 1. 02 R1n= 8773 wherein RiRa indicate the radii of the individual surfaces counting from the front, D1D2 indicate the axial thicknesses of the individual elements and SiS: indicate the axial air separations between the components.

17. An optical objective having numerical data substantially as set forth in the following table:

Equivalent focal length 1.000 Relative aperture f/l.4

R f Relattiie Thickness or e. me' Abbe v Paf i Radlub separation inenn number dlsgrmn Dx= .0671 1.613 59.6 1.00 R2=+L 653 S1=0 Ra= 8284 Dz= 0597 1` 613 59. 6 1.00 R4=+1. 650

Sz= 002 R5=| 442 D3= 1474 1. 644 48. 3 1. 02 R6: ce

S3= 2033 Rs= 3482 De= 1474 l. 644 48. 3 1. 02 Rro= 4854 D1= .0813 1.644 48.3 1.02 Ru= 7544 wherein RiRa indicate the radii of the individual surfaces counting from the front, DiDz indicate the axial thicknesses of the individual elements and S152 indicate the axial air separations between the components.

ARTHUR WARMISHAM.

CHARLES GORRIE WYNNE. 

